(18,38.98),(29,58.23)
slope = (58.23 - 38.98) / (29 - 18) = 19.25 / 11 = 1.75
y = mx + b
slope(m) = 1.75
use either of ur points...(18,38.98)...x = 18 and y = 38.98
now sub and find b, the y int
38.98 = 1.75(18) + b
38.98 = 31.5 + b
38.98 - 31.5 = b
7.48 = b
so ur equation is : y = 1.75x + 7.48...where x is the amount of water used and y is the monthly cost
so the cost for 23 hcf.....sub in 23 for x
y = 1.75x + 7.48
y = 1.75(23) + 7.48
y = 40.25 + 7.48
y = $ 47.73 <===== ur answer
